Science of Water : Concepts and Applications

326 The Science of Water: Concepts and Applications
WATER SOURCE CALCULATIONS
Water source calculations covered in this section apply to wells and pond/lake storage capacity.
Specifi c well calculations discussed include well drawdown, well yield, specifi c yield, well casing
disinfection, and deep-well turbine pump capacity.
Well Drawdown
Drawdown is the drop in the level of water in a well when water is being pumped. Drawdown is
usually measured in feet or meters.
One of the most important reasons for measuring drawdown is to make sure that the source
water is adequate and not being depleted. The data that are collected to calculate drawdown can
indicate if the water supply is slowly declining. Early detection can give the system time to explore
alternative sources, establish conservation measures, or obtain any special funding that may be
needed to get a new water source.
Well drawdown, the difference between the pumping water level and the static water level, is
calculated as follows:
Drawdown, ft pumping water level, ftstatic water level, ft
(10.1)
Example 10.1
Problem:
The static water level for a well is 70 ft. If the pumping water level is 90 ft, what is the drawdown?
Solution:
Drawdown, ft pumping water level, ftstatic water level, ft
90 ft 70
ft
20 ft
Example 10.2
Problem:
The static water level of a well is 122 ft. The pumping water level is determined using the sounding
line. The air pressure applied to the sounding line is 4.0 psi and the length of the sounding line is
180 ft. What is the drawdown?
Solution:
First calculate the water depth in the sounding line and the pumping water level:
1. Water depth in sounding line = (4.0 psi) (2.31 ft /psi) = 9.2 ft
2. Pumping water level = 180 ft – 9.2 ft = 170.8 ft
Then calculate drawdown as usual:
Well Yield
Drawdown, ft pumping water level, ftstatic water level, ft
170.8 ft 122
ft
48.8 ft
Well yield is the volume of water per unit of time that is produced from the well pumping. Usually
well yield is measured in terms of gallons per minute (gpm) or gallons per hour (gph). Sometimes
large fl ows are measured in cubic feet per second (cfs).